Finding Feature
Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information,
see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented
in this module, and to see a list of the releases in which each feature is supported, see the feature information table at
the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature
Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Information About Configuring IP Unicast Routing

This module describes how to
configure IP Version 4 (IPv4) unicast routing on the switch.

Note

On switches running the LAN
base feature, static routing on VLANs is supported only with this release.

A switch stack operates and appears as a single router to the rest of the routers in the network. Basic routing functions like static routing and the Routing Information Protocol (RIP), are available with both the IP Base feature set and the IP Services feature set. To use advanced routing features and other routing protocols, you must have the IP Services feature set enabled on the standalone
switch or on the active switch.

Note

In addition to IPv4 traffic, you can also enable IP Version 6 (IPv6) unicast routing and configure interfaces to forward IPv6
trafficif the switch or switch stack is running the IP Base or IP Services feature set
.

Information About IP Routing

In some network environments,
VLANs are associated with individual networks or subnetworks. In an IP network,
each subnetwork is mapped to an individual VLAN. Configuring VLANs helps
control the size of the broadcast domain and keeps local traffic local.
However, network devices in different VLANs cannot communicate with one another
without a Layer 3 device (router) to route traffic between the VLAN, referred
to as inter-VLAN routing. You configure one or more routers to route traffic to
the appropriate destination VLAN.

Figure 1. Routing Topology
Example. This figure shows
a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20.
The router has an interface in each VLAN.

When Host A in VLAN 10 needs
to communicate with Host B in VLAN 10, it sends a packet addressed to that
host. Switch A forwards the packet directly to Host B, without sending it to
the router.

When Host A sends a packet to
Host C in VLAN 20, Switch A forwards the packet to the router, which receives
the traffic on the VLAN 10 interface. The router checks the routing table,
finds the correct outgoing interface, and forwards the packet on the VLAN 20
interface to Switch B. Switch B receives the packet and forwards it to Host C.

Types of Routing

Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination.

Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing
is secure and uses little bandwidth, but does not automatically respond to changes in the network, such as link failures,
and therefore, might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability.

Switches running the LAN base
feature set support 16 user-configured static routes, in addition to any
default routes used for the management interface. The LAN base image supports
static routing only on SVIs.

Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic. There are two
types of dynamic routing protocols:

Routers using distance-vector protocols maintain routing tables with distance values of networked resources, and periodically
pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes.
These protocols are easy to configure and use.

Routers using link-state protocols maintain a complex database of network topology, based on the exchange of link-state advertisements
(LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time required
to respond to these changes. Link-state protocols respond quickly to topology changes, but require greater bandwidth and more
resources than distance-vector protocols.

Distance-vector protocols supported by the switch are Routing Information Protocol (RIP), which uses a single distance metric
(cost) to determine the best path and Border Gateway Protocol (BGP), which adds a path vector mechanism. The switch also supports
the Open Shortest Path First (OSPF) link-state protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features
to traditional Interior Gateway Routing Protocol (IGRP) to improve efficiency.

Note

On a switch or switch stack, the supported protocols are determined by the software running on the active switch. If the active switch is running the IP base feature set, only default routing, static routing and RIP are supported. If the switch is running the LAN base feature set, you can configure 16 static routes on SVIs. All other routing protocols
require the IP services feature set.

IP Routing and Switch Stacks

A switch stack appears to the
network as a single switch, regardless of which switch in the stack is
connected to a routing peer.

The active switch performs
these functions:

It initializes and configures
the routing protocols.

It sends routing protocol
messages and updates to other routers.

It processes routing protocol
messages and updates received from peer routers.

It generates, maintains, and
distributes the distributed Cisco Express Forwarding (dCEF) database to all
stack members. The routes are programmed on all switches in the stack bases on
this database.

The MAC address of the active
switch is used as the router MAC address for the whole stack, and all outside
devices use this address to send IP packets to the stack.

All IP packets that require
software forwarding or processing go through the CPU of the active switch.

Stack members perform these
functions:

They act as routing standby
switches, ready to take over in case they are elected as the new active switch
if the active switch fails.

They program the routes into
hardware.

If a active switch fails, the stack detects that
the active switch is down and elects one of the stack members to be the new
active switch. During this period, except for a momentary interruption, the
hardware continues to forward packets with no active protocols.

However, even though the switch stack maintains the hardware identification after a failure, the routing protocols on the
router neighbors might flap during the brief interruption before the active switch restarts. Routing protocols such as OSPF
and EIGRP need to recognize neighbor transitions. The router uses two levels of nonstop forwarding (NSF) to detect a switchover, to continue forwarding network traffic, and
to recover route information from peer devices:

NSF-aware routers tolerate neighboring router failures. After the neighbor router restarts, an NSF-aware router supplies information
about its state and route adjacencies on request.

NSF-capable routers support NSF. When they detect a active switch change, they rebuild routing information from NSF-aware
or NSF-capable neighbors and do not wait for a restart.

The switch stack supports NSF-capable routing for OSPF and EIGRP.

Upon election, the new active
switch performs these functions:

It starts generating,
receiving, and processing routing updates.

It builds routing tables,
generates the CEF database, and distributes it to stack members.

It uses its MAC address as
the router MAC address. To notify its network peers of the new MAC address, it
periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply
with the new router MAC address.

Note

If you configure the
persistent MAC address feature on the stack and the active switch changes, the
stack MAC address does not change for the configured time period. If the
previous active switch rejoins the stack as a member switch during that time
period, the stack MAC address remains the MAC address of the previous active
switch.

It attempts to determine the
reachability of every proxy ARP entry by sending an ARP request to the proxy
ARP IP address and receiving an ARP reply. For each reachable proxy ARP IP
address, it generates a gratuitous ARP reply with the new router MAC address.
This process is repeated for 5 minutes after a new active switch election.

Note

When a active switch is running the IP services feature set, the stack can run all supported protocols, including Open Shortest
Path First (OSPF), and Enhanced IGRP (EIGRP) and Border Gateway Protocol (BGP). If the active switch fails and the new elected active switch is running the IP base or LAN base feature set, these protocols
will no longer run in the stack.

Caution

Partitioning of the switch
stack into two or more stacks might lead to undesirable behavior in the
network.

If the switch is reloaded, then all the ports on that switch go down and there is a loss of traffic for the interfaces involved
in routing, despite NSF/SSO capability.

Classless Routing

By default, classless
routing behavior is enabled on the
Device
when it is configured to route. With classless routing, if a router receives
packets for a subnet of a network with no default route, the router forwards
the packet to the best supernet route. A supernet consists of contiguous blocks
of Class C address spaces used to simulate a single, larger address space and
is designed to relieve the pressure on the rapidly depleting Class B address
space.

In the figure, classless
routing is enabled. When the host sends a packet to 120.20.4.1, instead of
discarding the packet, the router forwards it to the best supernet route. If
you disable classless routing and a router receives packets destined for a
subnet of a network with no network default route, the router discards the
packet.

Figure 2. IP Classless
Routing

In the figure , the router in
network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and
128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no
network default route, the router discards the packet.

Figure 3. No IP Classless
Routing

To prevent the
Device
from forwarding packets destined for unrecognized subnets to the best supernet
route possible, you can disable classless routing behavior.

Address Resolution

You can control
interface-specific handling of IP by using address resolution. A device using
IP can have both a local address or MAC address, which uniquely defines the
device on its local segment or LAN, and a network address, which identifies the
network to which the device belongs.

Note

In a switch stack, network communication uses a single MAC address and the IP address of the stack.

The local address or MAC address is known as a
data link address because it is contained in the data link layer (Layer 2)
section of the packet header and is read by data link (Layer 2) devices. To
communicate with a device on Ethernet, the software must learn the MAC address
of the device. The process of learning the MAC address from an IP address is
called
address resolution. The process of learning the
IP address from the MAC address is called
reverse address resolution.

The
Device
can use these forms of address resolution:

Address Resolution Protocol (ARP) is used to
associate IP address with MAC addresses. Taking an IP address as input, ARP
learns the associated MAC address and then stores the IP address/MAC address
association in an ARP cache for rapid retrieval. Then the IP datagram is
encapsulated in a link-layer frame and sent over the network. Encapsulation of
IP datagrams and ARP requests or replies on IEEE 802 networks other than
Ethernet is specified by the Subnetwork Access Protocol (SNAP).

Proxy ARP helps hosts with no routing tables learn
the MAC addresses of hosts on other networks or subnets. If the
Device
(router) receives an ARP request for a host that is not on the same interface
as the ARP request sender, and if the router has all of its routes to the host
through other interfaces, it generates a proxy ARP packet giving its own local
data link address. The host that sent the ARP request then sends its packets to
the router, which forwards them to the intended host.

The
Device
also uses the Reverse Address Resolution Protocol (RARP), which functions the
same as ARP does, except that the RARP packets request an IP address instead of
a local MAC address. Using RARP requires a RARP server on the same network
segment as the router interface. Use the
ip rarp-server address
interface configuration command to identify the server.

Proxy ARP

Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to
communicate with hosts on other networks or subnets. The host assumes that all hosts are on the same local Ethernet and that
they can use ARP to learn their MAC addresses. If a Device receives an ARP request for a host that is not on the same network as the sender, the Device evaluates whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC
address, and the host that sent the request sends the packet to the Device, which forwards it to the intended host. Proxy ARP treats all networks as if they are local, and performs ARP requests for
every IP address.

ICMP Router Discovery Protocol

Router discovery allows the
Device
to dynamically learn about routes to other networks using ICMP router discovery
protocol (IRDP). IRDP allows hosts to locate routers. When operating as a
client, the
Device
generates router discovery packets. When operating as a host, the
Device
receives router discovery packets. The
Device
can also listen to Routing Information Protocol (RIP) routing updates and use
this information to infer locations of routers. The
Device
does not actually store the routing tables sent by routing devices; it merely
keeps track of which systems are sending the data. The advantage of using IRDP
is that it allows each router to specify both a priority and the time after
which a device is assumed to be down if no further packets are received.

Each device discovered
becomes a candidate for the default router, and a new highest-priority router
is selected when a higher priority router is discovered, when the current
default router is declared down, or when a TCP connection is about to time out
because of excessive retransmissions.

UDP Broadcast Packets and Protocols

User Datagram Protocol (UDP) is an IP host-to-host
layer protocol, as is TCP. UDP provides a low-overhead, connectionless session
between two end systems and does not provide for acknowledgment of received
datagrams. Network hosts occasionally use UDP broadcasts to find address,
configuration, and name information. If such a host is on a network segment
that does not include a server, UDP broadcasts are normally not forwarded. You
can remedy this situation by configuring an interface on a router to forward
certain classes of broadcasts to a helper address. You can use more than one
helper address per interface.

You can specify a UDP
destination port to control which UDP services are forwarded. You can specify
multiple UDP protocols. You can also specify the Network Disk (ND) protocol,
which is used by older diskless Sun workstations and the network security
protocol SDNS.

By default, both UDP and ND forwarding are enabled if a helper address has been defined for an interface.

Broadcast Packet Handling

After configuring an IP
interface address, you can enable routing and configure one or more routing
protocols, or you can configure the way the
Device
responds to network broadcasts. A broadcast is a data packet destined for all
hosts on a physical network. The
Device
supports two kinds of broadcasting:

A directed broadcast
packet is sent to a specific network or series of networks. A directed
broadcast address includes the network or subnet fields.

A flooded broadcast packet is
sent to every network.

Note

You can also limit broadcast,
unicast, and multicast traffic on Layer 2 interfaces by using the
storm-control interface configuration command to
set traffic suppression levels.

Routers provide some
protection from broadcast storms by limiting their extent to the local cable.
Bridges (including intelligent bridges), because they are Layer 2 devices,
forward broadcasts to all network segments, thus propagating broadcast storms.
The best solution to the broadcast storm problem is to use a single broadcast
address scheme on a network. In most modern IP implementations, you can set the
address to be used as the broadcast address. Many implementations, including
the one in the
Device,
support several addressing schemes for forwarding broadcast messages.

IP Broadcast Flooding

You can allow IP
broadcasts to be flooded throughout your internetwork in a controlled fashion
by using the database created by the bridging STP. Using this feature also
prevents loops. To support this capability, bridging must be configured on each
interface that is to participate in the flooding. If bridging is not configured
on an interface, it still can receive broadcasts. However, the interface never
forwards broadcasts it receives, and the router never uses that interface to
send broadcasts received on a different interface.

Packets that are forwarded to
a single network address using the IP helper-address mechanism can be flooded.
Only one copy of the packet is sent on each network segment.

To be considered for
flooding, packets must meet these criteria. (Note that these are the same
conditions used to consider packet forwarding using IP helper addresses.)

The packet must be a
MAC-level broadcast.

The packet must be an
IP-level broadcast.

The packet must be a TFTP,
DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by the
ip forward-protocol udp global configuration
command.

The time-to-live (TTL) value
of the packet must be at least two.

A flooded UDP datagram is
given the destination address specified with the
ip broadcast-address interface configuration
command on the output interface. The destination address can be set to any
address. Thus, the destination address might change as the datagram propagates
through the network. The source address is never changed. The TTL value is
decremented.

When a flooded UDP datagram
is sent out an interface (and the destination address possibly changed), the
datagram is handed to the normal IP output routines and is, therefore, subject
to access lists, if they are present on the output interface.

In the
Device, the majority of packets are forwarded
in hardware; most packets do not go through the
Device CPU. For those packets that do go to
the CPU, you can speed up spanning tree-based UDP flooding by a factor of about
four to five times by using turbo-flooding. This feature is supported over
Ethernet interfaces configured for ARP encapsulation.

How to Configure IP Routing

By default, IP routing is disabled on the Device, and you must enable it before routing can take place.

In the following
procedures, the specified interface must be one of these Layer 3 interfaces:

A routed port: a physical port configured as a
Layer 3 port by using the
no switchport interface configuration command.

A switch virtual interface
(SVI): a VLAN interface created by using the
interface vlan vlan_id global
configuration command and by default a Layer 3 interface.

Note

On enabling ip routing, the VLAN configured as SVI will also learn broadcast ARP requests which are not self destined.

An EtherChannel port channel in Layer 3 mode: a port-channel logical interface created by using the interface port-channel port-channel-number global configuration command and binding the Ethernet interface into the channel group.

Note

The switch does not support
tunnel interfaces for unicast routed traffic.

All Layer 3 interfaces on
which routing will occur must have IP addresses assigned to them.

Note

A Layer 3 switch can have an IP address assigned to
each routed port and SVI.

The number of routed ports and SVIs that you can configure is limited to 128, exceeding the recommended number and volume
of features being implemented might impact CPU utilization because of hardware limitations.

Configuring routing consists of several main
procedures:

To support VLAN interfaces, create and configure VLANs on the Device or switch stack, and assign VLAN membership to Layer 2 interfaces. For more information, see the "Configuring VLANs” chapter.

Configure Layer 3 interfaces.

Enable IP routing on the
switch.

Assign IP addresses to the
Layer 3 interfaces.

Enable selected routing
protocols on the switch.

Configure routing protocol
parameters (optional).

How to Configure IP Addressing

A required task
for configuring IP routing is to assign IP addresses to Layer 3 network
interfaces to enable the interfaces and allow communication with the hosts on
those interfaces that use IP. The following sections describe how to configure
various IP addressing features. Assigning IP addresses to the interface is
required; the other procedures are optional.

If a helper address is defined or User Datagram Protocol (UDP) flooding is configured, UDP forwarding is enabled on default
ports.

Any-local-broadcast: Disabled.

Spanning Tree Protocol (STP): Disabled.

Turbo-flood: Disabled.

IP helper address

Disabled.

IP host

Disabled.

IRDP

Disabled.

Defaults when enabled:

Broadcast IRDP advertisements.

Maximum interval between advertisements: 600 seconds.

Minimum interval between advertisements: 0.75 times max interval

Preference: 0.

IP proxy ARP

Enabled.

IP routing

Disabled.

IP subnet-zero

Disabled.

Assigning IP Addresses to Network Interfaces

An IP address
identifies a location to which IP packets can be sent. Some IP addresses are
reserved for special uses and cannot be used for host, subnet, or network
addresses. RFC 1166, “Internet Numbers,” contains the official description of
IP addresses.

An interface can have one
primary IP address. A mask identifies the bits that denote the network number
in an IP address. When you use the mask to subnet a network, the mask is
referred to as a subnet mask. To receive an assigned network number, contact
your Internet service provider.

Example:

Using Subnet Zero

Subnetting with a subnet
address of zero is strongly discouraged because of the problems that can arise
if a network and a subnet have the same addresses. For example, if network
131.108.0.0 is subnetted as 255.255.255.0, subnet zero would be written as
131.108.0.0, which is the same as the network address.

You can use the all ones
subnet (131.108.255.0) and even though it is discouraged, you can enable the
use of subnet zero if you need the entire subnet space for your IP address.

Use the
no ip subnet-zero global configuration
command to restore the default and disable the use of subnet zero.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ip
subnet-zero

Example:

Device(config)# ip subnet-zero

Enables the use of
subnet zero for interface addresses and routing updates.

Step 4

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 5

show running-config

Example:

Device# show running-config

Verifies your entries.

Step 6

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Disabling Classless Routing

To prevent the
Device
from forwarding packets destined for unrecognized subnets to the best supernet
route possible, you can disable classless routing behavior.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

no ip
classless

Example:

Device(config)#no ip classless

Disables classless
routing behavior.

Step 4

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 5

show running-config

Example:

Device# show running-config

Verifies your entries.

Step 6

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Configuring Address Resolution Methods

Defining a Static ARP Cache

ARP and other address resolution protocols
provide dynamic mapping between IP addresses and MAC addresses. Because most
hosts support dynamic address resolution, you usually do not need to specify
static ARP cache entries. If you must define a static ARP cache entry, you can
do so globally, which installs a permanent entry in the ARP cache that the
Device
uses to translate IP addresses into MAC addresses. Optionally, you can also
specify that the
Device
respond to ARP requests as if it were the owner of the specified IP address. If
you do not want the ARP entry to be permanent, you can specify a timeout period
for the ARP entry.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

arp ip-address hardware-address
type

Example:

Device(config)# ip 10.1.5.1 c2f3.220a.12f4 arpa

Associates an IP
address with a MAC (hardware) address in the ARP cache, and specifies
encapsulation type as one of these:

Proxy ARP

Proxy ARP is enabled by default. To enable it after it has been disabled, see the “Enabling Proxy ARP” section. Proxy ARP
works as long as other routers support it.

Default Gateway

Another method for
locating routes is to define a default router or default gateway. All non-local
packets are sent to this router, which either routes them appropriately or
sends an IP Control Message Protocol (ICMP) redirect message back, defining
which local router the host should use. The
Device
caches the redirect messages and forwards each packet as efficiently as
possible. A limitation of this method is that there is no means of detecting
when the default router has gone down or is unavailable.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ip
default-gateway ip-address

Example:

Device(config)# ip default gateway 10.1.5.1

Sets up a default
gateway (router).

Step 4

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 5

show ip
redirects

Example:

Device# show ip redirects

Displays the
address of the default gateway router to verify the setting.

Step 6

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

ICMP Router Discovery Protocol (IRDP)

The only required task for
IRDP routing on an interface is to enable IRDP processing on that interface.
When enabled, the default parameters apply.

You can optionally change any
of these parameters. If you change the
maxadvertinterval value, the
holdtime and
minadvertinterval values also change, so it is
important to first change the
maxadvertinterval value, before manually changing
either the
holdtime or
minadvertinterval values.

Example:

This command
allows for compatibility with Sun Microsystems Solaris, which requires IRDP
packets to be sent out as multicasts. Many implementations cannot receive these
multicasts; ensure end-host ability before using this command.

Step 6

ip irdp
holdtime seconds

Example:

Device(config-if)# ip irdp holdtime 1000

(Optional) Sets
the IRDP period for which advertisements are valid. The default is three times
the
maxadvertinterval value. It must be greater than
maxadvertinterval and cannot be greater than 9000
seconds. If you change the
maxadvertinterval value, this value also changes.

Step 7

ip irdp
maxadvertinterval seconds

Example:

Device(config-if)# ip irdp maxadvertinterval 650

(Optional) Sets
the IRDP maximum interval between advertisements. The default is 600 seconds.

Step 8

ip irdp
minadvertinterval seconds

Example:

Device(config-if)# ip irdp minadvertinterval 500

(Optional) Sets
the IRDP minimum interval between advertisements. The default is 0.75 times the
maxadvertinterval . If you change the
maxadvertinterval , this value changes to the new
default (0.75 of
maxadvertinterval ).

Step 9

ip irdp
preference number

Example:

Device(config-if)# ip irdp preference 2

(Optional) Sets a
device IRDP preference level. The allowed range is –231 to 231. The default is
0. A higher value increases the router preference level.

Step 10

ip irdp address address [number]

Example:

Device(config-if)# ip irdp address 10.1.10.10

(Optional)
Specifies an IRDP address and preference to proxy-advertise.

Enabling Directed Broadcast-to-Physical Broadcast
Translation

By default, IP
directed broadcasts are dropped; they are not forwarded. Dropping IP-directed
broadcasts makes routers less susceptible to denial-of-service attacks.

You can enable forwarding of
IP-directed broadcasts on an interface where the broadcast becomes a physical
(MAC-layer) broadcast. Only those protocols configured by using the
ip forward-protocol global configuration command
are forwarded.

You can specify an access list to control which broadcasts are forwarded. When an access list is specified, only those IP
packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts. For more
information on access lists, see the “Configuring ACLs" chapter in the Security section.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/2

Enters interface
configuration mode, and specifies the interface to configure.

Step 4

ip directed-broadcast
[access-list-number]

Example:

Device(config-if)# ip directed-broadcast 103

Enables directed
broadcast-to-physical broadcast translation on the interface. You can include
an access list to control which broadcasts are forwarded. When an access list,
only IP packets permitted by the access list can be translated.

Note

The
ip directed-broadcast interface configuration
command can be configured on a VPN routing/forwarding(VRF) interface and is VRF
aware. Directed broadcast traffic is routed only within the VRF.

Step 5

exit

Example:

Device(config-if)# exit

Returns to global
configuration mode.

Step 6

ip forward-protocol
{udp [port] |
nd |
sdns }

Example:

Device(config)# ip forward-protocol nd

Specifies which
protocols and ports the router forwards when forwarding broadcast packets.

Example:

Example:

Specifies which
protocols the router forwards when forwarding broadcast packets.

Step 7

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 8

show ip interface [interface-id]

Example:

Device# show ip interface gigabitethernet 1/0/1

Verifies the
configuration on the interface or all interfaces.

Step 9

show running-config

Example:

Device# show running-config

Verifies your entries.

Step 10

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Establishing an IP Broadcast Address

The most popular IP
broadcast address (and the default) is an address consisting of all ones
(255.255.255.255). However, the
Device
can be configured to generate any form of IP broadcast address.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/1

Enters interface
configuration mode, and specifies the interface to configure.

Step 4

ip
broadcast-address ip-address

Example:

Device(config-if)# ip broadcast-address 128.1.255.255

Enters a
broadcast address different from the default, for example 128.1.255.255.

Step 5

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 6

show ip interface [interface-id]

Example:

Device# show ip interface

Verifies the
broadcast address on the interface or all interfaces.

Step 7

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Flooding IP Broadcasts

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ip
forward-protocol spanning-tree

Example:

Device(config)# ip forward-protocol spanning-tree

Uses the bridging
spanning-tree database to flood UDP datagrams.

Step 4

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 5

show running-config

Example:

Device# show running-config

Verifies your entries.

Step 6

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Step 7

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 8

ip
forward-protocol turbo-flood

Example:

Device(config)# ip forward-protocol turbo-flood

Uses the
spanning-tree database to speed up flooding of UDP datagrams.

Step 9

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 10

show running-config

Example:

Device# show running-config

Verifies your entries.

Step 11

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Monitoring and Maintaining IP Addressing

When the contents of a particular
cache, table, or database have become or are suspected to be invalid, you can
remove all its contents by using the
clear privileged EXEC commands. The Table lists
the commands for clearing contents.

Table 2. Commands to Clear Caches,
Tables, and Databases

clear
arp-cache

Clears the IP ARP cache and
the fast-switching cache.

clear host {name |
* }

Removes one or all entries
from the hostname and the address cache.

clear ip route {network [mask] |
* }

Removes one or more routes
from the IP routing table.

You can display specific
statistics, such as the contents of IP routing tables, caches, and databases;
the reachability of nodes; and the routing path that packets are taking through
the network. The Table lists the privileged EXEC commands for displaying IP
statistics.

Table 3. Commands to Display Caches,
Tables, and Databases

show
arp

Displays the entries in the
ARP table.

show
hosts

Displays the default domain
name, style of lookup service, name server hosts, and the cached list of
hostnames and addresses.

show ip
aliases

Displays IP addresses mapped
to TCP ports (aliases).

show ip
arp

Displays the IP ARP cache.

show ip interface
[interface-id]

Displays the IP status of
interfaces.

show ip
irdp

Displays IRDP values.

show ip
masks address

Displays the masks used for
network addresses and the number of subnets using each mask.

show ip
redirects

Displays the address of a
default gateway.

show ip route [address [mask]] | [protocol]

Displays the current state of
the routing table.

show ip route
summary

Displays the current state of
the routing table in summary form.

How to Configure IP Unicast Routing

Enabling IP Unicast Routing

By default, the
Device
is in Layer 2 switching mode and IP routing is disabled. To use the Layer 3
capabilities of the
Device,
you must enable IP routing.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ip routing

Example:

Device(config)# ip routing

Enables IP routing.

Step 4

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 5

show running-config

Example:

Device# show running-config

Verifies your entries.

Step 6

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Example of Enabling IP
Routing

This example shows how to enable IP routingusing RIP as the routing protocol
:

What to Do Next

You can now set up parameters for the selected routing protocols as described in these sections:

RIP

OSPF,

EIGRP

BGP

Unicast Reverse Path Forwarding

Protocol-Independent Features (optional)

Information About RIP

The Routing Information Protocol (RIP) is an
interior gateway protocol (IGP) created for use in small, homogeneous networks.
It is a distance-vector routing protocol that uses broadcast User Datagram
Protocol (UDP) data packets to exchange routing information. The protocol is
documented in RFC 1058. You can find detailed information about RIP in
IP Routing Fundamentals,
published by Cisco Press.

Note

RIP is supported in the IP BaseNetwork Essentials feature set.

Using RIP, the
Device
sends routing information updates (advertisements) every 30 seconds. If a
router does not receive an update from another router for 180 seconds or more,
it marks the routes served by that router as unusable. If there is still no
update after 240 seconds, the router removes all routing table entries for the
non-updating router.

RIP uses hop counts to rate the value of
different routes. The hop count is the number of routers that can be traversed
in a route. A directly connected network has a hop count of zero; a network
with a hop count of 16 is unreachable. This small range (0 to 15) makes RIP
unsuitable for large networks.

If the router has a default
network path, RIP advertises a route that links the router to the pseudonetwork
0.0.0.0. The 0.0.0.0 network does not exist; it is treated by RIP as a network
to implement the default routing feature. The
Device
advertises the default network if a default was learned by RIP or if the router
has a gateway of last resort and RIP is configured with a default metric. RIP
sends updates to the interfaces in specified networks. If an interface’s
network is not specified, it is not advertised in any RIP update.

Summary Addresses and Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon
mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised
by a router on any interface from which that information originated. This feature usually optimizes communication among multiple
routers, especially when links are broken.

How to Configure RIP

Default RIP Configuration

Table 4. Default RIP
Configuration

Feature

Default Setting

Auto summary

Enabled.

Default-information originate

Disabled.

Default metric

Built-in; automatic metric
translations.

IP RIP authentication
key-chain

No authentication.

Authentication mode: clear
text.

IP RIP triggered

Disabled

IP split horizon

Varies with media.

Neighbor

None defined.

Network

None specified.

Offset list

Disabled.

Output delay

0 milliseconds.

Timers basic

Update: 30 seconds.

Invalid: 180 seconds.

Hold-down: 180 seconds.

Flush: 240 seconds.

Validate-update-source

Enabled.

Version

Receives RIP Version 1 and 2
packets; sends Version 1 packets.

Configuring Basic RIP Parameters

To configure RIP, you enable
RIP routing for a network and optionally configure other parameters. On the
Device,
RIP configuration commands are ignored until you configure the network number.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ip
routing

Example:

Device(config)# ip routing

Enables IP
routing. (Required only if IP routing is disabled.)

Step 4

router
rip

Example:

Device(config)# router rip

Enables a RIP
routing process, and enter router configuration mode.

Step 5

network network number

Example:

Device(config-router)# network 12.0.0.0

Associates a
network with a RIP routing process. You can specify multiple
network commands. RIP routing updates are sent and
received through interfaces only on these networks.

Note

You must
configure a network number for the RIP commands to take effect.

Step 6

neighbor ip-address

Example:

Device(config-router)# neighbor 10.2.5.1

(Optional)
Defines a neighboring router with which to exchange routing information. This
step allows routing updates from RIP (normally a broadcast protocol) to reach
nonbroadcast networks.

Example:

update—The time between sending routing updates.
The default is 30 seconds.

invalid—The timer after which a route is declared
invalid. The default is 180 seconds.

holddown—The time before a route is removed from
the routing table. The default is 180 seconds.

flush—The amount of time for which routing updates
are postponed. The default is 240 seconds.

Step 9

version {1 |
2 }

Example:

Device(config-router)# version 2

(Optional)
Configures the switch to receive and send only RIP Version 1 or RIP Version 2
packets. By default, the switch receives Version 1 and 2 but sends only Version
1. You can also use the interface commands
ip rip {send |
receive }
version 1 |
2 |
1 2 } to control
what versions are used for sending and receiving on interfaces.

Example:

Device(config-router)# output-delay 8

(Optional) Adds interpacket delay for RIP updates sent. By default, packets in a multiple-packet RIP update have no delay
added between packets. If you are sending packets to a lower-speed device, you can add an interpacket delay in the range of
8 to 50 milliseconds.

Step 12

end

Example:

Device(config-router)# end

Returns to privileged EXEC mode.

Step 13

show ip
protocols

Example:

Device# show ip protocols

Verifies your
entries.

Step 14

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Configuring RIP Authentication

RIP Version 1 does not
support authentication. If you are sending and receiving RIP Version 2 packets,
you can enable RIP authentication on an interface. The key chain specifies the
set of keys that can be used on the interface. If a key chain is not
configured, no authentication is performed, not even the default.

The
Device
supports two modes of authentication on interfaces for which RIP authentication
is enabled: plain text and MD5. The default is plain text.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/1

Enters interface
configuration mode, and specifies the interface to configure.

Step 4

ip rip
authentication key-chain name-of-chain

Example:

Device(config-if)# ip rip authentication key-chain trees

Enables RIP
authentication.

Step 5

ip rip authentication mode {text |
md5 }

Example:

Device(config-if)# ip rip authentication mode md5

Configures the
interface to use plain text authentication (the default) or MD5 digest
authentication.

Step 6

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 7

show running-config

Example:

Device# show running-config

Verifies your entries.

Step 8

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Configuring Summary Addresses and Split Horizon

Note

In general, disabling split
horizon is not recommended unless you are certain that your application
requires it to properly advertise routes.

If you want to configure an interface running
RIP to advertise a summarized local IP address pool on a network access server
for dial-up clients, use the
ip summary-address rip interface configuration
command.

Example:

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Configuring Split Horizon

Routers connected to
broadcast-type IP networks and using distance-vector routing protocols normally
use the split-horizon mechanism to reduce the possibility of routing loops.
Split horizon blocks information about routes from being advertised by a router
on any interface from which that information originated. This feature can
optimize communication among multiple routers, especially when links are
broken.

Note

In general, we do not
recommend disabling split horizon unless you are certain that your application
requires it to properly advertise routes.

Procedure

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/1

Enters interface
configuration mode, and specifies the interface to configure.

Step 4

ip address ip-address
subnet-mask

Example:

Device(config-if)# ip address 10.1.1.10 255.255.255.0

Configures the IP
address and IP subnet.

Step 5

no ip split-horizon

Example:

Device(config-if)# no ip split-horizon

Disables split horizon on the interface.

Step 6

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 7

show ip
interface interface-id

Example:

Device# show ip interface gigabitethernet 1/0/1

Verifies your
entries.

Step 8

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Configuration Example for Summary Addresses and Split Horizon

In this example, the major net is 10.0.0.0. The summary address 10.2.0.0 overrides the autosummary address of 10.0.0.0 so
that 10.2.0.0 is advertised out interface Gigabit Ethernet port 2, and 10.0.0.0 is not advertised. In the example, if the
interface is still in Layer 2 mode (the default), you must enter a no switchport interface configuration command before entering the ip address interface configuration command.

The Cisco implementation
conforms to the OSPF Version 2 specifications with these key features:

Definition of stub areas is
supported.

Routes learned through any IP
routing protocol can be redistributed into another IP routing protocol. At the
intradomain level, this means that OSPF can import routes learned through EIGRP
and RIP. OSPF routes can also be exported into RIP.

Plain text and MD5
authentication among neighboring routers within an area is supported.

OSPF typically requires coordination among many
internal routers, area border routers (ABRs) connected to multiple areas, and
autonomous system boundary routers (ASBRs). The minimum configuration would use
all default parameter values, no authentication, and interfaces assigned to
areas. If you customize your environment, you must ensure coordinated
configuration of all routers.

Note

It is not recommended to use OSPF aggressive timers. An OSPF hello timer of less than five seconds is considered aggressive.
OSPF and other routing protocols are handled at normal priority and sub second scheduling under high CPU usage conditions
in not guaranteed.

BFD control packets are handled with high priority by a separate queue and bfd packets are processed in a high priority thread.
BFD is preferred over OSPF for faster convergence.

OSPF Nonstop Forwarding

OSPF NSF Awareness

The IP-services feature set supports OSPF NSF Awareness for IPv4.
When the neighboring router is NSF-capable, the Layer 3 Device continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in
a router crashing and the backup RP taking over, or while the primary RP is manually reloaded for a non-disruptive software
upgrade.

This feature cannot be
disabled.

OSPF NSF Capability

The IP services feature set supports the OSPFv2 NSF IETF format in addition to the OSPFv2 NSF Cisco format that is supported in earlier releases. For
information about this feature, see : NSF—OSPF (RFC 3623 OSPF Graceful Restart).

The IP-services feature set also supports OSPF NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack master change.
When a stack master change occurs in an OSPF NSF-capable stack, the new stack master must do two things to resynchronize its
link-state database with its OSPF neighbors:

Release the available OSPF neighbors on the network without resetting the neighbor relationship.

Reacquire the contents of the link-state database for the network.

After a stack master change, the new master sends an OSPF NSF signal to neighboring NSF-aware devices. A device recognizes
this signal to mean that it should not reset the neighbor relationship with the stack. As the NSF-capable stack master receives
signals from other routes on the network, it begins to rebuild its neighbor list.

When the neighbor relationships are reestablished, the NSF-capable stack master resynchronizes its database with its NSF-aware
neighbors, and routing information is exchanged between the OSPF neighbors. The new stack master uses this routing information
to remove stale routes, to update the routing information database (RIB), and to update the forwarding information base (FIB)
with the new information. The OSPF protocols then fully converge.

Note

OSPF NSF requires that all
neighbor networking devices be NSF-aware. If an NSF-capable router discovers
non-NSF aware neighbors on a network segment, it disables NSF capabilities for
that segment. Other network segments where all devices are NSF-aware or
NSF-capable continue to provide NSF capabilities.

Use the
nsf OSPF routing configuration command to enable
OSPF NSF routing. Use the
show ip ospf privileged EXEC command to verify
that it is enabled.

OSPF Area Parameters

You can optionally configure several OSPF area parameters. These parameters include authentication for password-based protection
against unauthorized access to an area, stub areas, and not-so-stubby-areas (NSSAs). Stub areas are areas into which information
on external routes is not sent. Instead, the area border router (ABR) generates a default external route into the stub area
for destinations outside the autonomous system (AS). An NSSA does not flood all LSAs from the core into the area, but can
import AS external routes within the area by redistribution.

Route summarization is the consolidation of advertised addresses into a single summary route to be advertised by other areas.
If network numbers are contiguous, you can use the area range router configuration command to configure the ABR to advertise a summary route that covers all networks in the range.

Other OSPF Parameters

You can optionally configure
other OSPF parameters in router configuration mode.

Route summarization: When redistributing routes from other
protocols. Each route is advertised individually in an external LSA. To help
decrease the size of the OSPF link state database, you can use the
summary-address router configuration command to advertise a
single router for all the redistributed routes included in a specified network
address and mask.

Virtual links: In OSPF, all areas must be
connected to a backbone area. You can establish a virtual link in case of a
backbone-continuity break by configuring two Area Border Routers as endpoints
of a virtual link. Configuration information includes the identity of the other
virtual endpoint (the other ABR) and the nonbackbone link that the two routers
have in common (the transit area). Virtual links cannot be configured through a
stub area.

Default route: When you
specifically configure redistribution of routes into an OSPF routing domain,
the route automatically becomes an autonomous system boundary router (ASBR).
You can force the ASBR to generate a default route into the OSPF routing
domain.

Domain Name Server (DNS)
names for use in all OSPF
show privileged EXEC command displays makes it
easier to identify a router than displaying it by router ID or neighbor ID.

Default Metrics: OSPF
calculates the OSPF metric for an interface according to the bandwidth of the
interface. The metric is calculated as
ref-bw divided by bandwidth, where
ref is 10 by default, and bandwidth (bw)
is specified by the
bandwidth interface configuration command. For
multiple links with high bandwidth, you can specify a larger number to
differentiate the cost on those links.

Administrative distance is a rating of the
trustworthiness of a routing information source, an integer between 0 and 255,
with a higher value meaning a lower trust rating. An administrative distance of
255 means the routing information source cannot be trusted at all and should be
ignored. OSPF uses three different administrative distances: routes within an
area (interarea), routes to another area (interarea), and routes from another
routing domain learned through redistribution (external). You can change any of
the distance values.

Passive interfaces: Because
interfaces between two devices on an Ethernet represent only one network
segment, to prevent OSPF from sending hello packets for the sending interface,
you must configure the sending device to be a passive interface. Both devices
can identify each other through the hello packet for the receiving interface.

Route calculation timers: You can configure the delay time
between when OSPF receives a topology change and when it starts the shortest
path first (SPF) calculation and the hold time between two SPF calculations.

Log neighbor changes: You can
configure the router to send a syslog message when an OSPF neighbor state
changes, providing a high-level view of changes in the router.

LSA Group Pacing

The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing, check-summing, and aging functions
for more efficient router use. This feature is enabled by default with a 4-minute default pacing interval, and you will not
usually need to modify this parameter. The optimum group pacing interval is inversely proportional to the number of LSAs the
router is refreshing, check-summing, and aging. For example, if you have approximately 10,000 LSAs in the database, decreasing
the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval
to 10 to 20 minutes might benefit you slightly.

Loopback Interfaces

OSPF uses the highest IP address configured on the interfaces as its router ID. If this interface is down or removed, the
OSPF process must recalculate a new router ID and resend all its routing information out its interfaces. If a loopback interface
is configured with an IP address, OSPF uses this IP address as its router ID, even if other interfaces have higher IP addresses.
Because loopback interfaces never fail, this provides greater stability. OSPF automatically prefers a loopback interface over
other interfaces, and it chooses the highest IP address among all loopback interfaces.

How to Configure OSPF

Default OSPF Configuration

Table 5. Default OSPF
Configuration

Feature

Default Setting

Interface parameters

Cost:
1.

Retransmit interval: 5
seconds.

Transmit delay: 1 second.

Priority: 1.

Hello interval: 10 seconds.

Dead interval: 4 times the
hello interval.

No authentication.

No password specified.

MD5 authentication disabled.

Area

Authentication type: 0 (no
authentication).

Default cost: 1.

Range: Disabled.

Stub: No stub area defined.

NSSA: No NSSA area defined.

Auto cost

100 Mb/s.

Default-information originate

Disabled. When enabled, the
default metric setting is 10, and the external route type default is Type 2.

Default metric

Built-in, automatic metric
translation, as appropriate for each routing protocol.

Distance OSPF

dist1 (all routes within an
area): 110. dist2 (all routes from one area to another): 110. and dist3 (routes
from other routing domains): 110.

OSPF database filter

Disabled. All outgoing
link-state advertisements (LSAs) are flooded to the interface.

Configuring Basic OSPF Parameters

To enable OSPF, create an OSPF routing process, specify the range of IP addresses to associate with the routing process, and
assign area IDs to be associated with that range. For switches running the IP services image, you can configure either the Cisco OSPFv2 NSF format or the IETF OSPFv2 NSF format.

Procedure

Command or Action

Purpose

Step 1

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 2

router
ospf process-id

Example:

Device(config)# router ospf 15

Enables OSPF
routing, and enter router configuration mode. The process ID is an internally
used identification parameter that is locally assigned and can be any positive
integer. Each OSPF routing process has a unique value.

Note

OSPF for Routed Access supports only one OSPFv2 and one OSPFv3 instance with a maximum number of 1000 dynamically learned
routes.

Example:

(Optional) Enables IETF NSF operations for OSPF. The restart-interval keyword specifies the length of the graceful restart interval, in seconds. The range is from 1 to 1800. The default is 120.

Note

Enter the command in Step 3 or Step 4, and go to Step 5.

Step 5

network address wildcard-mask
area area-id

Example:

Device(config)# network 10.1.1.1 255.240.0.0 area 20

Define an
interface on which OSPF runs and the area ID for that interface. You can use
the wildcard-mask to use a single command to define one or more multiple
interfaces to be associated with a specific OSPF area. The area ID can be a
decimal value or an IP address.

Step 6

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 7

show ip
protocols

Example:

Device# show ip protocols

Verifies your
entries.

Step 8

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Configuring OSPF Interfaces

You can use the
ip ospf interface configuration commands to modify
interface-specific OSPF parameters. You are not required to modify any of these
parameters, but some interface parameters (hello interval, dead interval, and
authentication key) must be consistent across all routers in an attached
network. If you modify these parameters, be sure all routers in the network
have compatible values.

Example:

Example:

(Optional)
Explicitly specifies the cost of sending a packet on the interface.

Step 4

ip ospf
retransmit-interval seconds

Example:

Device(config-if)# ip ospf transmit-interval 10

(Optional)
Specifies the number of seconds between link state advertisement transmissions.
The range is 1 to 65535 seconds. The default is 5 seconds.

Step 5

ip ospf
transmit-delay seconds

Example:

Device(config-if)# ip ospf transmit-delay 2

(Optional) Sets
the estimated number of seconds to wait before sending a link state update
packet. The range is 1 to 65535 seconds. The default is 1 second.

Step 6

ip ospf
priority number

Example:

Device(config-if)# ip ospf priority 5

(Optional) Sets
priority to help find the OSPF designated router for a network. The range is
from 0 to 255. The default is 1.

Step 7

ip ospf
hello-interval seconds

Example:

Device(config-if)# ip ospf hello-interval 12

(Optional) Sets
the number of seconds between hello packets sent on an OSPF interface. The
value must be the same for all nodes on a network. The range is 1 to 65535
seconds. The default is 10 seconds.

Step 8

ip ospf
dead-interval seconds

Example:

Device(config-if)# ip ospf dead-interval 8

(Optional) Sets
the number of seconds after the last device hello packet was seen before its
neighbors declare the OSPF router to be down. The value must be the same for
all nodes on a network. The range is 1 to 65535 seconds. The default is 4 times
the hello interval.

Step 9

ip ospf
authentication-key key

Example:

Device(config-if)# ip ospf authentication-key password

(Optional) Assign
a password to be used by neighboring OSPF routers. The password can be any
string of keyboard-entered characters up to 8 bytes in length. All neighboring
routers on the same network must have the same password to exchange OSPF
information.

Step 10

ip ospf message
digest-key keyid
md5 key

Example:

Device(config-if)# ip ospf message digest-key 16 md5 your1pass

(Optional)
Enables MDS authentication.

keyid—An identifier from 1 to 255.

key—An alphanumeric password of up to 16 bytes.

Step 11

ip ospf
database-filter all out

Example:

Device(config-if)# ip ospf database-filter all out

(Optional) Block
flooding of OSPF LSA packets to the interface. By default, OSPF floods new LSAs
over all interfaces in the same area, except the interface on which the LSA
arrives.

Step 12

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 13

show ip ospf interface [interface-name]

Example:

Device# show ip ospf interface

Displays
OSPF-related interface information.

Step 14

show ip ospf
neighbor detail

Example:

Device# show ip ospf neighbor detail

Displays NSF
awareness status of neighbor switch. The output matches one of these examples:

Options is
0x52

LLS Options is
0x1 (LR)

When both of
these lines appear, the neighbor switch is NSF aware.

Options is
0x42—This means the neighbor switch is not NSF aware.

Step 15

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Configuring OSPF Area Parameters

Before you begin

Note

The OSPF
area router configuration commands are all
optional.

Procedure

Command or Action

Purpose

Step 1

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 2

router
ospf process-id

Example:

Device(config)# router ospf 109

Enables OSPF
routing, and enter router configuration mode.

Step 3

area area-idauthentication

Example:

Device(config-router)# area 1 authentication

(Optional) Allow
password-based protection against unauthorized access to the identified area.
The identifier can be either a decimal value or an IP address.

Step 4

area area-idauthentication
message-digest

Example:

Device(config-router)# area 1 authentication message-digest

(Optional)
Enables MD5 authentication on the area.

Step 5

area area-idstub [no-summary ]

Example:

Device(config-router)# area 1 stub

(Optional) Define
an area as a stub area. The
no-summary keyword prevents an ABR from sending
summary link advertisements into the stub area.

Example: Configuring Basic OSPF Parameters

Information About EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the IGRP. EIGRP uses the same distance vector algorithm and
distance information as IGRP; however, the convergence properties and the operating efficiency of EIGRP are significantly
improved.

The convergence technology employs an algorithm referred to as the Diffusing Update Algorithm (DUAL), which guarantees loop-free
operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronize
at the same time. Routers that are not affected by topology changes are not involved in recomputations.

IP EIGRP provides increased network width. With RIP, the largest possible width of your network is 15 hops. Because the EIGRP
metric is large enough to support thousands of hops, the only barrier to expanding the network is the transport-layer hop
counter. EIGRP increments the transport control field only when an IP packet has traversed 15 routers and the next hop to
the destination was learned through EIGRP. When a RIP route is used as the next hop to the destination, the transport control
field is incremented as usual.

EIGRP Components

EIGRP has these four basic
components:

Neighbor discovery and recovery is the process that routers
use to dynamically learn of other routers on their directly attached networks.
Routers must also discover when their neighbors become unreachable or
inoperative. Neighbor discovery and recovery is achieved with low overhead by
periodically sending small hello packets. As long as hello packets are
received, the Cisco IOS software can learn that a neighbor is alive and
functioning. When this status is determined, the neighboring routers can
exchange routing information.

The reliable transport protocol is responsible for guaranteed,
ordered delivery of EIGRP packets to all neighbors. It supports intermixed
transmission of multicast and unicast packets. Some EIGRP packets must be sent
reliably, and others need not be. For efficiency, reliability is provided only
when necessary. For example, on a multiaccess network that has multicast
capabilities (such as Ethernet), it is not necessary to send hellos reliably to
all neighbors individually. Therefore, EIGRP sends a single multicast hello
with an indication in the packet informing the receivers that the packet need
not be acknowledged. Other types of packets (such as updates) require
acknowledgment, which is shown in the packet. The reliable transport has a
provision to send multicast packets quickly when there are unacknowledged
packets pending. Doing so helps ensure that convergence time remains low in the
presence of varying speed links.

The DUAL finite state machine embodies the decision process
for all route computations. It tracks all routes advertised by all neighbors.
DUAL uses the distance information (known as a metric) to select efficient,
loop-free paths. DUAL selects routes to be inserted into a routing table based
on feasible successors. A successor is a neighboring router used for packet
forwarding that has a least-cost path to a destination that is guaranteed not
to be part of a routing loop. When there are no feasible successors, but there
are neighbors advertising the destination, a recomputation must occur. This is
the process whereby a new successor is determined. The amount of time it takes
to recompute the route affects the convergence time. Recomputation is
processor-intensive; it is advantageous to avoid recomputation if it is not
necessary. When a topology change occurs, DUAL tests for feasible successors.
If there are feasible successors, it uses any it finds to avoid unnecessary
recomputation.

The protocol-dependent modules are responsible for network
layer protocol-specific tasks. An example is the IP EIGRP module, which is
responsible for sending and receiving EIGRP packets that are encapsulated in
IP. It is also responsible for parsing EIGRP packets and informing DUAL of the
new information received. EIGRP asks DUAL to make routing decisions, but the
results are stored in the IP routing table. EIGRP is also responsible for
redistributing routes learned by other IP routing protocols.

Note

To enable EIGRP, the Device or stack master must be running the IP services feature set.

EIGRP Nonstop Forwarding

EIGRP NSF Awareness

The IP-services feature set supports EIGRP NSF Awareness for IPv4. When the neighboring router is NSF-capable, the Layer 3 Device continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in
a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software
upgrade. This feature cannot be disabled.

EIGRP NSF Capability

The IP services feature set supports EIGRP Cisco NSF routing to speed up convergence and to eliminate traffic loss after a stack master change.

The IP services feature set
also supports EIGRP NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack master
change. When an EIGRP NSF-capable stack master restarts or a new stack master starts up and NSF restarts, the Device has no neighbors, and the topology table is empty. The Device must bring up the interfaces, reacquire neighbors, and rebuild the topology and routing tables without interrupting the traffic
directed toward the Device stack. EIGRP peer routers maintain the routes learned from the new stack master and continue forwarding traffic through the
NSF restart process.

To prevent an adjacency reset
by the neighbors, the new stack master uses a new Restart (RS) bit in the EIGRP
packet header to show the restart. When the neighbor receives this, it
synchronizes the stack in its peer list and maintains the adjacency with the
stack. The neighbor then sends its topology table to the stack master with the
RS bit set to show that it is NSF-aware and is aiding the new stack master.

If at least one of the stack
peer neighbors is NSF-aware, the stack master receives updates and rebuilds its
database. Each NSF-aware neighbor sends an end of table (EOT) marker in the
last update packet to mark the end of the table content. The stack master
recognizes the convergence when it receives the EOT marker, and it then begins
sending updates. When the stack master has received all EOT markers from its
neighbors or when the NSF converge timer expires, EIGRP notifies the routing
information database (RIB) of convergence and floods its topology table to all
NSF-aware peers.

EIGRP Stub Routing

The EIGRP stub routing feature, available in all feature sets, reduces resource utilization by moving routed traffic closer to the end user.

Note

The IP Base
feature set contains EIGRP stub routing capability, which only advertises connected or summary routes from the routing
tables to other device in the network.
The device uses EIGRP stub routing at the access layer to eliminate the need for other types of routing advertisements. For enhanced capability and complete EIGRP routing, the device must be running the IP Base feature set.On a device running the IP base feature set, if you try to configure multi-VRF-CE and EIGRP stub routing at the same time, the configuration
is not allowed. IPv6 EIGRP stub routing is not supported with the IP base feature set.

In a network using EIGRP stub routing, the only allowable route for IP traffic to the user is through a device that is configured with EIGRP stub routing. The device sends the routed traffic to interfaces that are configured as user interfaces or are connected to other devices.

When using EIGRP stub routing, you need to configure the distribution and remote routers to use EIGRP and to configure only
the device as a stub. Only specified routes are propagated from the device. The device responds to all queries for summaries, connected routes, and routing updates.

Any neighbor that receives a
packet informing it of the stub status does not query the stub router for any
routes, and a router that has a stub peer does not query that peer. The stub
router depends on the distribution router to send the proper updates to all
peers.

In the figure given below, device B is configured as an EIGRP stub router. Devicees A and C are connected to the rest of the WAN. Device B advertises connected, static, redistribution, and summary routes to Device A and C. Device B does not advertise any routes learned from Device A (and the reverse).

Figure 4. EIGRP Stub Router
Configuration

How to Configure EIGRP

To create an EIGRP routing process, you must enable EIGRP and associate networks. EIGRP sends updates to the interfaces in
the specified networks. If you do not specify an interface network, it is not advertised in any EIGRP update.

Note

If you have routers on your network that are configured for IGRP, and you want to change to EIGRP, you must designate transition
routers that have both IGRP and EIGRP configured. In these cases, perform Steps 1 through 3 in the next section and also see
the “Configuring Split Horizon” section. You must use the same AS number for routes to be automatically redistributed.

Example:

(Optional)
Specifies the time period during which the key can be received.

The start-time and
end-time syntax can be either
hh:mm:ss Month date year or
hh:mm:ss date Month year. The default is forever
with the default
start-time and the earliest acceptable date as
January 1, 1993. The default
end-time and
duration is
infinite .

Step 10

send-lifetime start-time {infinite |
end-time |
duration seconds}

Example:

(Optional)
Specifies the time period during which the key can be sent.

The start-time and
end-time
syntax can be either
hh:mm:ss Month date
year or
hh:mm:ss date Month
year. The default is forever with the default
start-time
and the earliest acceptable date as January 1, 1993. The default
end-time and
duration is
infinite .

Step 11

end

Example:

Device(config)# end

Returns to
privileged EXEC mode.

Step 12

show key
chain

Example:

Device# show key chain

Displays
authentication key information.

Step 13

copy running-config
startup-config

Example:

Device# copy running-config startup-config

(Optional) Saves your entries
in the configuration file.

Monitoring and Maintaining EIGRP

You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics. The table given below
lists the privileged EXEC commands for deleting neighbors and displaying statistics.

Displays the number of
packets sent and received for all or a specified EIGRP process.

Information About BGP

The Border Gateway Protocol (BGP) is an exterior gateway protocol used to set up an interdomain routing system that guarantees
the loop-free exchange of routing information between autonomous systems. Autonomous systems are made up of routers that operate
under the same administration and that run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries
and that interconnect by using an Exterior Gateway Protocol (EGP). BGP Version 4 is the standard EGP for interdomain routing
in the Internet. The protocol is defined in RFCs 1163, 1267, and 1771.

BGP Network Topology

Routers that belong to the same autonomous system
(AS) and that exchange BGP updates run internal BGP (IBGP), and routers that
belong to different autonomous systems and that exchange BGP updates run
external BGP (EBGP). Most configuration commands are the same for configuring
EBGP and IBGP. The difference is that the routing updates are exchanged either
between autonomous systems (EBGP) or within an AS (IBGP). The figure given
below shows a network that is running both EBGP and IBGP.

Figure 5. EBGP, IBGP, and Multiple
Autonomous Systems

Before exchanging information
with an external AS, BGP ensures that networks within the AS can be reached by
defining internal BGP peering among routers within the AS and by redistributing
BGP routing information to IGPs that run within the AS, such as IGRP and OSPF.

Routers that run a BGP
routing process are often referred to as BGP speakers. BGP uses the
Transmission Control Protocol (TCP) as its transport protocol (specifically
port 179). Two BGP speakers that have a TCP connection to each other for
exchanging routing information are known as peers or neighbors. In the above
figure, Routers A and B are BGP peers, as are Routers B and C and Routers C and
D. The routing information is a series of AS numbers that describe the full
path to the destination network. BGP uses this information to construct a
loop-free map of autonomous systems.

The network has these
characteristics:

Routers A and B are running
EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are
directly connected and that the IBGP peers are not. As long as there is an IGP
running that allows the two neighbors to reach one another, IBGP peers do not
have to be directly connected.

All BGP speakers within an AS
must establish a peer relationship with each other. That is, the BGP speakers
within an AS must be fully meshed logically. BGP4 provides two techniques that
reduce the requirement for a logical full mesh: confederations and route
reflectors.

AS 200 is a transit AS for AS
100 and AS 300—that is, AS 200 is used to transfer packets between AS 100 and
AS 300.

BGP peers initially exchange
their full BGP routing tables and then send only incremental updates. BGP peers
also exchange keepalive messages (to ensure that the connection is up) and
notification messages (in response to errors or special conditions).

In BGP, each route consists of
a network number, a list of autonomous systems that information has passed
through (the autonomous system path), and a list of other path attributes. The
primary function of a BGP system is to exchange network reachability
information, including information about the list of AS paths, with other BGP
systems. This information can be used to determine AS connectivity, to prune
routing loops, and to enforce AS-level policy decisions.

A router or
Device
running Cisco IOS does not select or use an IBGP route unless it has a route
available to the next-hop router and it has received synchronization from an
IGP (unless IGP synchronization is disabled). When multiple routes are
available, BGP bases its path selection on attribute values. See the
“Configuring BGP Decision Attributes” section for information about BGP
attributes.

BGP Version 4 supports classless interdomain routing
(CIDR) so you can reduce the size of your routing tables by creating aggregate
routes, resulting in supernets. CIDR eliminates the concept of network classes
within BGP and supports the advertising of IP prefixes.

Nonstop Forwarding Awareness

The BGP NSF Awareness feature is supported for IPv4 in the IP services feature set.
. To enable this feature with BGP routing, you need to enable Graceful Restart. When the neighboring router is NSF-capable,
and this feature is enabled, the Layer 3 Device continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in
a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software
upgrade.

Information About BGP Routing

To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP must completely recognize
the relationships with its neighbors, you must also specify a BGP neighbor.

BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same AS; external neighbors are
in different autonomous systems. External neighbors are usually adjacent to each other and share a subnet, but internal neighbors
can be anywhere in the same AS.

The switch supports the use of private AS numbers, usually assigned by service providers and given to systems whose routes
are not advertised to external neighbors. The private AS numbers are from 64512 to 65535. You can configure external neighbors
to remove private AS numbers from the AS path by using the neighbor remove-private-as router configuration command. Then when an update is passed to an external neighbor, if the AS path includes private AS numbers,
these numbers are dropped.

If your AS will be passing traffic through it from another AS to a third AS, it is important to be consistent about the routes
it advertises. If BGP advertised a route before all routers in the network had learned about the route through the IGP, the
AS might receive traffic that some routers could not yet route. To prevent this from happening, BGP must wait until the IGP
has propagated information across the AS so that BGP is synchronized with the IGP. Synchronization is enabled by default.
If your AS does not pass traffic from one AS to another AS, or if all routers in your autonomous systems are running BGP,
you can disable synchronization, which allows your network to carry fewer routes in the IGP and allows BGP to converge more
quickly.

Routing Policy Changes

Routing policies for a peer
include all the configurations that might affect inbound or outbound routing
table updates. When you have defined two routers as BGP neighbors, they form a
BGP connection and exchange routing information. If you later change a BGP
filter, weight, distance, version, or timer, or make a similar configuration
change, you must reset the BGP sessions so that the configuration changes take
effect.

There are two types of reset, hard
reset and soft reset. Cisco IOS Releases 12.1 and later support a soft reset
without any prior configuration. To use a soft reset without preconfiguration,
both BGP peers must support the soft route refresh capability, which is
advertised in the OPEN message sent when the peers establish a TCP session. A
soft reset allows the dynamic exchange of route refresh requests and routing
information between BGP routers and the subsequent re-advertisement of the
respective outbound routing table.

When soft reset generates
inbound updates from a neighbor, it is called dynamic inbound soft reset.

When soft reset sends a set
of updates to a neighbor, it is called outbound soft reset.

A soft inbound reset causes
the new inbound policy to take effect. A soft outbound reset causes the new
local outbound policy to take effect without resetting the BGP session. As a
new set of updates is sent during outbound policy reset, a new inbound policy
can also take effect.

The table given below lists
the advantages and disadvantages hard reset and soft reset.

Table 9. Advantages and Disadvantages
of Hard and Soft Resets

Type of Reset

Advantages

Disadvantages

Hard reset

No memory overhead

The prefixes in the BGP, IP,
and FIB tables provided by the neighbor are lost. Not recommended.

Outbound soft reset

No configuration, no storing
of routing table updates

Does not reset inbound
routing table updates.

Dynamic inbound soft reset

Does not clear the BGP
session and cache

Does not require storing of
routing table updates and has no memory overhead

Both BGP routers must support
the route refresh capability (in Cisco IOS Release 12.1 and later).

BGP Decision Attributes

When a BGP speaker receives
updates from multiple autonomous systems that describe different paths to the
same destination, it must choose the single best path for reaching that
destination. When chosen, the selected path is entered into the BGP routing
table and propagated to its neighbors. The decision is based on the value of
attributes that the update contains and other BGP-configurable factors.

When a BGP peer learns two EBGP paths for a
prefix from a neighboring AS, it chooses the best path and inserts that path in
the IP routing table. If BGP multipath support is enabled and the EBGP paths
are learned from the same neighboring autonomous systems, instead of a single
best path, multiple paths are installed in the IP routing table. Then, during
packet switching, per-packet or per-destination load-balancing is performed
among the multiple paths. The
maximum-paths router configuration command
controls the number of paths allowed.

These factors summarize the
order in which BGP evaluates the attributes for choosing the best path:

If the path specifies a
next hop that is inaccessible, drop the update. The BGP next-hop attribute,
automatically determined by the software, is the IP address of the next hop
that is going to be used to reach a destination. For EBGP, this is usually the
IP address of the neighbor specified by the
neighbor
remote-as router configuration command. You can disable next-hop
processing by using route maps or the
neighbor
next-hop-self router configuration command.

Prefer the path with the largest
weight (a Cisco proprietary parameter). The weight attribute is local to the
router and not propagated in routing updates. By default, the weight attribute
is 32768 for paths that the router originates and zero for other paths. Routes
with the largest weight are preferred. You can use access lists, route maps, or
the
neighbor
weight router configuration command to set weights.

Prefer the route with the
highest local preference. Local preference is part of the routing update and
exchanged among routers in the same AS. The default value of the local
preference attribute is 100. You can set local preference by using the
bgp default local-preference router configuration
command or by using a route map.

Prefer the route that was
originated by BGP running on the local router.

Prefer the route with the
shortest AS path.

Prefer the route with the
lowest origin type. An interior route or IGP is lower than a route learned by
EGP, and an EGP-learned route is lower than one of unknown origin or learned in
another way.

Prefer the route with the lowest multi -exit discriminator (MED)
metric attribute if the neighboring AS is the same for all routes considered.
You can configure the MED by using route maps or by using the
default-metric router configuration command. When
an update is sent to an IBGP peer, the MED is included.

Prefer the external (EBGP)
path over the internal (IBGP) path.

Prefer the route that can be
reached through the closest IGP neighbor (the lowest IGP metric). This means
that the router will prefer the shortest internal path within the AS to reach
the destination (the shortest path to the BGP next-hop).

If the following conditions
are all true, insert the route for this path into the IP routing table:

Both the best route and this
route are external.

Both the best route and this
route are from the same neighboring autonomous system.

Maximum-paths is enabled.

If multipath is not enabled,
prefer the route with the lowest IP address value for the BGP router ID. The
router ID is usually the highest IP address on the router or the loopback
(virtual) address, but might be implementation-specific.

Route Maps

Within BGP, route maps can be used to control and to modify routing information and to define the conditions by which routes
are redistributed between routing domains. Each route map has a name that identifies the route map (map tag) and an optional sequence number.

BGP Filtering

You can filter BGP advertisements by using AS-path filters, such
as the
as-path access-list global configuration command
and the
neighbor filter-list router configuration command.
You can also use access lists with the
neighbor distribute-list router configuration
command. Distribute-list filters are applied to network numbers. See the
“Controlling Advertising and Processing in Routing Updates” section for
information about the
distribute-list command.

You can use route maps on a
per-neighbor basis to filter updates and to modify various attributes. A route
map can be applied to either inbound or outbound updates. Only the routes that
pass the route map are sent or accepted in updates. On both inbound and
outbound updates, matching is supported based on AS path, community, and
network numbers. Autonomous system path matching requires the
match as-path access-list